The invention relates to an optical fiber amplifier device and more specifically to a double stage amplifier structure with different amplifying fibers. More specifically the invention also relates to a communications system which uses the invention of optical fiber amplifier device.
An optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal had the input to such an amplifier is monochromatic the output also will be monochromatic with the same frequency. Conventional fiber amplifier comprises a gain medium such as a glass fiber core doped with an active material into which is coupled to input signal. Excitation occurs from the absorption of optical pumping energy by the core. The optical pumping energy is within the absorption band of the active material in the core, and when the optical signal propagates through the core, the absorbed pump energy causes amplification of the signal transmitted through the fiber core by stimulated emission. Optical amplifiers are typically used in a variety of applications including but not limited to amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems.
One typical example of a fiber amplifier is referred to as an erbium amplifier, and includes a silica fiber having a single-mode core doped with erbium. It is well known that an erbium optical fiber amplifier operating in its standard three level mode is capable when pumped at a wavelength of 980 nanometers of amplifying optical signals having a wavelength of 1550 nanometer. Since this wavelength is the lowest loss wavelength of conventional single-mode silica glass fibers erbium amplifiers are well suited for inclusion in fiber systems that propagate signals having wavelengths around 1550 nanometer.
In certain particular high-power ones it may be desirable to provide optical amplification using a double-clad fiber. A typical double-clad fiber has an inner core through which an optical signal is transmitted, an inner cladding surrounding the core that is of lower refractive index than the core and an outer cladding surrounding the inner cladding that has a lower refractive index than the inner cladding and is not absorbent. When using a double-clad fiber for optical amplification it is known that the optical pumping energy need not be coupled directly into the core where it will be absorbed by amplification purposes but may be coupled into the inner cladding where it propagates in various reflective trajectories through the cladding until it intersects the core. Once contacting the core pump energy is absorbed and provides stored energy in the core for stimulates emission amplification of the optical signal.
One well-known problem with the use of double-clad fibers for optical amplifiers is the fact that among the transmission modes of the pumping energy through the inner cladding are a number which never intersect the core. Because a conventional double-clad fiber has a cylindrical core surrounded by an annular inner cladding it is possible for a number of helical modes to exist within the inner cladding which travel through the inner cladding without ever intersecting the core. Since these modes never intersect the core the pump energy is not absorbed and does not contribute to the amplification of the optical signal. This had led to attempts to reduce the helical spatial modes within the inner cladding of a double-clad fiber.
The advantage of using a double-clad fiber amplifier is that a very high level of pump energy can be coupled to the fiber multi mode inner cladding. The pumping modules can included a high power laser diodes or laser array or in some examples a group of fiber lasers. One prior art is the U.S. Pat. No. 6,081,369 disclosing a double-clad fiber amplifier pumped by a group of fiber lasers.
An other attempt to improve the efficiency of power conversion is the use of optical ring doped fiber. In the U.S. Pat. No. 5,970,198 a optical fiber ring structure is disclosed which is used to improve the gain spectrum of an optical amplifier. This fiber has a center core doped with germanium and erbium and a ring region also doped with erbium. The ring region formed around the center core region so as to be radially spaced a given distance from the center core region. Especially this dopant structure can absorb pumping light at different wavelength bands and improve the behavior of the amplifying fiber.
It is therefore an objective of the present invention to provide an optical amplifying device with a high efficiency of energy conversion from pumping energy to signal energy. In accordance with an aspect of the present invention there is provided an optical fiber amplifier device with an optical signal input and output comprising a first piece of amplifying fiber doped with lanthanide in a double-clad structure and a second piece amplifying fiber doped with lanthanide in ring structure and means for pumping the fiber pieces with at least one pump module.